Add 64bit expr values

[imrehorvath]

In the AVR Assembler (AVRASM2), "all expressions are 64 bits internally". (See spec. chapter 7. Expressions) For compatibility and for integer arithmetic required eg. for delay cycles calculation, the full 64 bit support is required. The current avra 1.4.2 implementation uses 32 bit integers internally for expressions which in turn causes eg. delay cycles calculation in macros to overflow and fail, limiting the usage seriously. This PR adds 64 bits integer support for the expression values. Regression tests also provided.